That depends on the job I want to do. But generally my selection is something like this.

1. Is it a short simple script: Bash
2. Longer script, then a more competent dynamic language like Perl/Python.
3. Backend, a strong typed compiled language, with as few runtime errors as possible. If it depends on some particular API, the language with good enough bindings.

Preferred backend language, Rust, since that have the least runtime errors, thanks to its strong typing and the great error handling. But I also use Go if it have better libs for what I do, or Java for situations where that is more suitable.